Longitudinal mode resonator filters, also called dual mode filters or DMS filters, are referred to below as surface wave filters referenced OFW filters that comprise two normal finger transducers arranged within reflectors according to the schematic plan view according to FIG. 1 and that are effective as input or, respectively, output transducer 2, 12. They have two-pole filter characteristics. When two DMS tracks A and B that are frequency-shifted relative to one another such that the high-frequency resonance of the low-frequency track B falls onto the low-frequency resonance of the high-frequency track A are interconnected as shown in FIG. 1, then what is referred to as a twin dual mode filter is obtained, this being distinguished by three-pole filter characteristics and by an enhanced edge steepness compared to the single track.
Given the filter structure according to FIG. 1, a respective bond wire at the side of the filter input and filter output IN or, respectively, OUT must be conducted at least across one connecting line, across one of the DMS tracks A, B in the worst case. This requires additional chip or, respectively, substrate area for the bond pads 5 connected to the transducers 2, 12 via interconnects 4 between the DMS tracks A, B and complicates the fabrication of such a filter.
It is also disadvantageous that the impedances of the filter can be made arbitrarily smaller by enlarging the aperture since disturbing transverse modes occur in this case.
Given symmetrical operation of the twin dual mode filter, over and above this, no optimum suppression of the electrical crosstalk is achieved since the geometry is not symmetrical.
When the DMS tracks are not optimally aligned relative to one another with respect to the resonant frequencies, then increased amplitude and group running time distortions occur in the filter transmission band. Such a de-adjustment is unavoidable in mass production due to the scatters of the manufacturing parameters and the inhomogeneities on the wafer.